Although protein serine proteinase inhibitors of the Kunitz, Kazal, and Bowman-Birk families make do with lock-and-key type interactions to inhibit target proteinases, serpins have now been shown by x-ray crystallography, NMR spectroscopy and FRET to employ a remarkable conformational change-based mechanism to inhibit some of the same serine proteinases. This mechanism involves changes to both the serpin and the proteinase, with insertion of the serpin reactive center loop (RCL) into the center of its own b-sheet A, and translocation of the proteinase over 70A to the distal end of the serpin from its initial docking position, with concomitant distortion of the proteinase active site and a "crushing" of a large portion of the proteinase upon it reaching its final location. Whereas these studies have provided a structural explanation of how serpins inhibit serine proteinases, they have not explained why such a complex, error-prone, mechanism is employed. An unproven assumption is that the conformational changes that occur in both serpin and proteinase are exploited subsequently for other downstream events, whether recognition and signaling via a receptor such as LRP, or else rendering the proteinase incapable of revival, through cleavage while in the serpin-proteinase complex. We propose three specific aims that together will enable evaluation of the importance of the full proteinase-translocating, proteinase-crushing sequence in expression of the functions that make serpins the preferred class of proteinase inhibitors under many circumstances. Specific Aim 1 will determine the structural requirements and functional consequences of serpin and proteinase binding to the receptor LRP. Specific Aim 2 will determine the structure of the mosquito serpin AFXa, its mechanism of inhibition of human factor Xa and the properties of the AFXa-Xa complex in relation to LRP binding and signaling. Specific Aim 3 will determine the conformational changes that occur in caspases, cathepsins and subtilisin-like proteinases upon formation of covalent complexes with serpins. We will use thermodynamic measurements of binding affinity and structural approaches of NMR spectroscopy, FRET and x-ray crystallography that we have successfully used in the previous grant period and, where appropriate, correlate our findings with functional consequences, through a collaboration with Dr. Dudley Strickland.